1. Field of the Invention
The present invention generally relates to a method and device for initializing an optical recording medium of a phase-change type.
2. Description of the Related Art
An optical recording medium, particularly an optical recording medium of a phase change type has an initial state (erased state) of a crystal phase. Generally, a recording layer has an amorphous phase after it is grown to a film. Hence, it is necessary to crystallize the grown film of the recording layer. This is an initializing step.
It is required that the initializing step initialize the grown film so that it is uniform, that is, so that the film has little variation in the reflection radio. It is also required to initialize the film as fast as possible.
In order to satisfy the above requirements, it has been proposed to utilize a high-power semiconductor laser diode of a broad-area type (see 5th Phase Change Recording Study Meeting, 1993, pp. 30). This paper proposes a method of projecting a hyperelliptic laser beam spot of 1 .mu.m to 100 .mu.m onto a disk to be initialized so that the beam is orthogonal to tracks formed on the disk. This method can initialize tens of tracks at one time.
Japanese Unexamined Patent Publication No. 7-52526 discloses that a light is split into two beams of an identical wavelength. The two beams are superimposed so as to form interference fringes, which are used to initialize the disk. Hence, it is possible to prevent a micro crack from being formed in a heat-resistance layer, so that the repetitive recording performance of the disk can be improved.
However, the above two conventional methods do not make the recording layer have an even reflection ratio. The method proposed in the former paper utilizes the high-power semiconductor laser diode of the broad-area type as a light source for initialization. Hence, as shown in "Laser Study", vol. 18, No. 8, pp. 555-pp. 572, a near-field pattern parallel to the junction interface of the laser (horizontal lateral mode) is trapezoidal, so that the intensity profile is not uniform inherently. The method of the latter paper utilizes the interference fringes, and it is thus impossible to inherently avoid unevenness of the reflection ratio.